1. Field of the Invention
The invention relates to an apparatus including a hollow cylindrical cutting knife (trepan) for cutting a corneal disc from a human eye.
2. Description of the Prior Art
In ophthalmology, it is known to transplant corneal tissue from a donor eye to a patients eye. Such operations, called perforating kerataoplastic, are performed in the case of scarring or disease of the cornea which lead to an irregular cornea outer surface, whereby the image formation on the retina by the cornea is distorted when the cornea is acting as the most strongly refracting lens element of the eye. There exist the possibility of obtaining the corneal disc which is to be transplanted either from a whole donor eye or from a cornea which has been preserved in the preserving solution. In the perforating kerataoplastic the cornea is installed in a concave support and a corneal disc to be transplanted is carved out with the aid of a round cutting knife, called a trepanning, from the posterior side of the endothelium of the cornea, towards the anterior, the epithelium, which abuts the support.
The trepanning on the eye of the patient is done by fixing the eye with a ring which is brought in upon the eutis vera or corium of the eye for stabilization of the cornea shape. Most recently, there have been conducted also some fixations with the aid of the vacuum ring lying against the eye. For the trepanning, a hand-held, hand-operated or motor-operated trephine is brought against the cornea and the corneal disc is cut out by rotation.
These methods have a serious drawback, in that it is not possible to obtained identical corneal discs from the donor eye on the one hand and the patient's eye in the other. This has various causes. The trephining operation is influenced by the elasticity and the resistance of the cornea to the cutting movement by the trephine. Moreover, the penetrating cuts, which are made from the front, to the posterior of the cornea are broader, i.e., larger in diameter, at the exit place than at the entrance location, so that the cutting surface is not perpendicular to the surface of the endothelium, but rather angled, or inclined. Thereby, one obtains in the trephining operation from the posterior side of a cornea a greater diameter at the anterior exterior surface of the cornea and conversely in the trephining operation on the patient eye one obtains a greater diameter on the inner side or surface of the cornea. The fitting together of the donated cornea disc in a patients' eyes is according to this method possible only by pressing together of the tissue.
Moreover, a free-hand guided trepan, such as it is used on the patient's eye, cannot by grinded exactly perpendicularly to the outer surface. Also, the eye moves during the trephining operation, and as the depth of cut increases, a lesser cutting resistance is encountered due to the lower opposition by the eye's inner pressure opposing the trepan as compared to the original pressure. The mobility of the eye leads easily to a departure of the trephine position from the desired perpendicular direction for the plane of the limbus of the cornea. An angular holding of the trepan has an equally adverse affect. A mathematical calculation of the affect on angled holding of the trepan yields, even with a departure of 10 degrees, a difference of more than 4 dioptres in the two corneal planes. The consequence of this is that even with well conducted trephining operations, with an angled departure of no more than 5 degrees from the perpendicular of the donor eye and an equal departure in the eye of the receiver, the effect of the angled portion values can become accumulative and produce the above indicated astigmatism. Angled departures of the trephine of 10 to 15 degrees from the vertical plane, which cause astigmatisms of 10 diopters, are therefore not rare.